The accreting plate boundary Ardoukoˆba Rift (northeast Africa) and the oceanic Rift Valley
The accreting plate boundary Ardoukoˆba Rift (northeast Africa) and the oceanic Rift Valley
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增生板块边界阿尔杜科巴裂谷（非洲东北部）和大洋裂谷
DOI：
10.1016/0012-821x(76)90205-3
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发表时间：
1976
影响因子：
5.3
通讯作者：
P. Tapponnier
中科院分区：
文献类型：
--
作者：
H. Needham;P. Choukroune;J. Cheminée;X. Pichon;J. Francheteau;P. Tapponnier
Ardoukoˆba裂谷位于法国领土阿法斯和伊萨斯(东北部非洲)的Ghoubbat-al-Kharab和Asal湖之间12公里处，从空中暴露给∼，具有固有的特征和区域背景，与以下论点一致：它是地壳增长的地点，其速度与沿裂谷的中大洋海脊发现的速度(∼2厘米/年)大致相同。阿尔杜科∼ˆba裂谷中心区宽11公里，内部起伏不到∼300米，位于裂谷最深部分上方800米的台阶状山脊之间。中心区较低的向内陡坡与狭窄的内地板接壤。典型的∼25-35公里宽的大洋裂谷的中心区可以有更大和更粗糙的起伏，宽度为∼8-16公里，但内部起伏为400米的深部区域的最大宽度与Ardoukoˆba裂谷(∼11公里)的相应区域的最大宽度大致相同。阿尔杜科ˆ巴裂谷的内底宽度从2公里到5公里不等，大洋裂谷的范围从不到1公里到∼9公里。两种背景下构造和火山作用的等价性尚未得到证明；但将大西洋36°50‘N附近著名地区的裂谷部分与Ardoukoˆba裂谷进行比较，鼓励使用后者的证据来补充关于大洋裂谷作为一个整体的火山作用和陡峭形成模式的论点。裂谷的内底是水平伸展的主要部位，没有垂直位移，垂直错动很少或没有积累的正断层作用，构造火山作用，可能进一步集中在狭窄的(∼1公里宽)裂隙带上。破坏并或多或少限制内底板有序生长的正断层作用可能是这样发生的，即成为新内底板的新地块相对于裂谷中线和假想地块的对称轴发生横向错动，说明中央正磁异常。此外，同构造期和构造后构造期火山作用以及年轻地壳任何时刻沿转换断层之间的部分距离的正常断层位移可能会引入额外的复杂性。因此，尽管开张速率在原则上总是等同于两个板块增加新地壳的总数，但这种假设是可疑的，即扩展速率(向一个板块或另一个板块增加地壳的速率)的概念必然可以精确地应用于裂谷的中央部分。更广泛地说，磁异常时间尺度上的插补扩散率的物理意义值得商榷。根据来自裂谷的证据，扩张率不必反映新地壳的单调添加，来自相反板块的相同推断年龄的岩石可能具有不同的成分。北大西洋在很长一段时间内的明显对称增长突显了这个问题。
The Ardoukoˆba Rift, subaerially exposed for ∼12 km between the Ghoubbat-al-Kharab and Lake Asal in the French Territory of the Afars and Issas (northeast Africa), has intrinsic features and a regional setting consistent with arguments that it is the site of crustal accretion at approximately the same rate as that found along the rifted Mid-Oceanic Ridge (∼2 cm/yr). The ∼11 km wide Central Zone of the Ardoukoˆba Rift has an internal relief of less than ∼300 m and is set between step-like ridges standing up to 800 m above the deepest part of the rift. The lower inward-facing scarps of the Central Zone border a narrow Inner Floor. The Central Zone of the typically ∼25–35 km wide oceanic Rift Valley can have a greater and rougher relief and has a width of ∼8–16 km, but deep areas with an internal relief of <400 m have a maximum width that is about the same as that of the corresponding area in the Ardoukoˆba Rift (∼11 km). The width of the Inner Floor of the Ardoukoˆba Rift varies from 2 to 5 km; in the oceanic Rift Valley the range is from less than 1 to ∼9 km. Equivalence of tectonic and volcanic processes in the two settings has not been demonstrated; but a comparison of a segment of the Rift Valley in the FAMOUS area near 36°50′N in the Atlantic with the Ardoukoˆba Rift encourages the tentative use of evidence from the latter to complement arguments about the pattern of vulcanism and scarp formation in the oceanic Rift Valley as a whole. The Inner Floor of the Rift Valley is the main site of horizontal extension without vertical displacements, of normal faulting that involves little or no accumulation of vertical offsets, and of constructional vulcanism, which may be further concentrated along narrow (∼1 km wide) fissured zones. The normal faulting that disrupts and constrains more or less orderly growth of the Inner Floor may happen in such a way that the new graben that become new Inner Floors are laterally offset with respect to the middle line of the Rift Valley and to the axis of symmetry of a hypothetical block accounting for the central positive magnetic anomaly.Additional complexities may be introduced by syntectonic and post-tectonic vulcanism, and by normal fault displacement at any one time of young crust along only part of the distance between transform faults. Thus, althoughopening ratecan always be equated in principle with total addition of new crust to the two plates, the assumption is suspect that the concept ofspreading rate(rate of addition of crust to one plate or the other) can necessarily be applied precisely to the central part of the Rift Valley. In more general terms, the physical meaning of interpolated spreading rates on the time scale of magnetic anomalies is worth questioning. On evidence from the Rift Valley, the spreading rates need not reflect monotonic additions of new crust, and rocks of the same inferred age from opposite plates may not have the same composition. The problem is highlighted by the apparently symmetrical growth of the North Atlantic over long periods of time.